equal vertices spacing when using displacement modifier
When converting a color (RGB) value to a scalar (just one value), you want to set all three channels of the RGB trio to the same value. Here, I think you changed the Value of the HSV trio describing the color. the exact conversion equation may vary from software to software, but here it should work like that.
For instance, a color of (R:0.2,G:0.2,B:0.2) can also be noted as (H:0,S:0,V:0.485). So the value is not linearly correlated to the mean of the RGB values.

The displacement using levels of 0.2 on the RGB channels instead of HSV :

How to convert rgb to hsv :

Source